Truncated through feeding thread rolling die

ABSTRACT

A through feed thread rolling die for rolling external threads on a cylindrical work piece has an external thread thereon with relieved starting and finishing sections, the starting relief providing flat crests which form a predetermined angle with the roll axis and taper to a diameter at the starting end less than the mean height of the fully formed threads. A modified version tapers the starting section at a larger angle than the predetermined angle of the crests thereby reducing the length of the starting section. The invention includes the method of metal movement caused by the die in the formation of the thread.

United States Patent Zook et al. [4 1 Mar. 28, 1972 s41 TRUNCATED THROUGH FEEDING 3,481,178 12/1969 Wilkins ..72/469 THREAD ROLLING DIE 2,828,493 4/1958 Koehler ..10/ 152 3,131,407 5/1964 Roberts ..l0/l52 [72] Inventors: Elmer S. Zook, Princeton; Paul W. Ronn,

Gardner, both of Mass. FOREIGN PATENTS OR APPLICATIONS 1 1 Assignee: Reed Rolled Thread Die Holden, 41,593 3/1932 France ..10/141 Mass.

[22] Filed: No 6, 1968 Prir nary Examiner-Charles W. Lanham Assistant ExammerE. M. Combs PP 773,851 Attorney-Chittick, Pfund, Birch, Samuels & Gauthier 52 us. c1 ..72/1o3, 72/89, 10/152 [571 ABSTRACT [51] CL 3/041 B211 3/02! B233 7/02 1 A through feed thread rolling die for rolling external threads [58] Field of Search ..10/2, 10, 11,24, 27 141, on a cylindrical work piece has an external thread thereon 10/ [521 152 T; 72/86 with relieved starting and finishing sections, the starting relief 103; 85/46 1 providing flat crests which form a predetermined angle with the roll axis and taper to a diameter at the starting end less [56] References cued than the mean height of the fully formed threads. A modified UNITED STATES PATENTS version tapers the starting section at a larger angle than the predetermined angle of the crests thereby reducing the length of the starting section. The invention includes the method of metal movement caused by the die in the formation of the thread. I

2 Claims, 6 Drawing Figures PATENTEDMAR28 m2 3,651,678

SHEET 3 [IF 3 W,Q ,M, Ma GQJBM TRUNCATED THROUGH FEEDING THREAD ROLLING DIE BACKGROUND OF THE INVENTION The field of this invention pertains to tools for rolling threads on work pieces as they pass between or through a cluster of such tools (usually two or three) which are rotated in unison to produce surface metal flow on the work piece to form a continuous thread configuration.

In the prior art, it is conventional practice to have a tapered entry section on thread rolling dies such that there is provided a converging opening between the dies that permits the work readily to enter therein. The rolling thread producing surfaces of the dies thus runs onto the work piece in a manner gradually to initiate the metal flow below the surface of the work piece that will raise portions corresponding to the crests of the finished thread on the work piece and correspondingly depress portions of the work piece corresponding to the roots of the finished thread. The usual conventional practice provides a starting section having a fully formed thread tapered both at the crest and the root according to some small and generally equal angle for both root and crest thereby to provide a converging opening to the final thread dimensions as the work piece enters the dies. This arrangement of the prior art, namely, the initial entry of the sharp edge of the die tooth followed by progressive deeper penetration of the same tooth form gives rise to a finished thread formation in the work in which there appears the disadvantage of a fine continuous crevice extending the length of the finished thread. This crevice is the inevitable result of metal flow in the work when fully formed tapered die threads are used in the starting section. A second disadvantage of the prior art dies involves frequent misalignment of the work relative to the axial path through the die cluster which results in what are commonly referred to as drunken threads.

Various arrangements in the prior art have been suggested for improving thread rolling operations, such as the patent to Erdelyi et al., US. Pat. No. 2,720,801, in which a set of dies are inclined to the axis of the work and provided with a root and crest contour to make full contact with the threads formed in the work. While this patent suggests maintaining a constant root diameter for the threadrolling die in conjunction with a tapered run-on section in which there are rounded crests on the initial thread turns, the initial alignment of the work in the die paths and the control of subsurface flow in the work due to broad area contact with fiat crests on the die threads is not suggested. The patent to Koehler US. Pat. No. 2,828,493 shows a tap including a roller die for producing an internal thread in which the metal is compressed rather than redistributed. Roberts US. Pat. No. 3,131,407 shows a thread swaging tap which slides with respect to the internal threads which are progressively developed by compressing the metal in the wall of the hole.

SUMMARY OF THE INVENTION In the present invention a thread rolling die is adapted to be rotatably supported in a cluster exteriorly of a cylindrical work piece with the roll axis either parallel to or skewed from the axis of the work. The rolls have at the tapered leading end a flat crest configuration to assure precision initial alignment of the work with the passline of the die cluster. The die configuration substantially eliminates the misalignment problems heretofore encountered which resulted in the undesirable production of drunken threads." It has also been found that the flat crest of progressively decreasing axial length on the initial threads of the die develops a subsurface flow of metal, as distinguished from compression of the metal which, as the diameter of the crests on the thread roll increase toward the final dimension of the dwell section of the die, produces a metal deformation which develops the crest of the external thread on the work into a fully formed thread without the production of the previously referred to crack of crevice characteristic in the prior art. Thus the crest of the developing thread on the work flows uniformly into the root of the die thread to produce a fully formed thread which has a crest that completely fills the root of the die and has a structural integrity of solid metal in contrast to the prior art formation of opposed folds with a crevice remaining therebetween. This flow is accomplished in the preferred form of the invention by maintaining the root diameter of the die roll uniform in the starting relief section while the crests of the starting section are tapered at a small uniform angle to produce flat crests of diminishing width as the full diameter of the dwell section of the die roll is approached.

In the alternate embodiment which provides a shortened starting relief section, this flattened crest configuration is retained, but the overall taper of the envelope of the flat crests is formed at a greater run-on angle thereby reducing the total number of threads in the run-on section when compared to the full length embodiment which has the same angle for the fiat thread crests and the taper angle.

It is, accordingly, the principal object of the present invention to provide a through-feed thread rolling die which can be supported in a cluster of two or more identical dies exteriorly of the cylindrical work piece and each having an axis of rotation either parallel to the passline axis of the work through the cluster, or uniformly skewed with respect to the passline, thus to provide improved starting section thread configurations that permit accurate initial alignment and improved metal deformation of the work to produce mechanically sound threads that do not suffer from the presence of a minute crack or crevice in the thread crest.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a view partly in section of a thread rolling die having a helical thread made in accordance with the invention shown in relation to an incoming cylindrical work piece.

FIG. 2 is an enlarged fragmentary view of a portion of the work piece and die shown in FIG. 1, representing the initial development of the thread on the work piece. FIG. 3 is a view similar to FIG. I of a modified form of the invention having a truncated starting relief section.

FIG. 4 is an enlarged view of and substantially in agreement with a portion of FIG. 3 showing the initial development of threads on the work piece and the relation between the angles of the flat crests and the envelope of the crest in the starting relief section.

FIG. 5 is a mechanical detail showing the relation of the angles a and B in the embodiment of FIGS. 3 and 4.

FIG. 6 is a view partially in section of a thread rolling die having a plurality of annular threads made in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, a thread rolling die generally indicated as 11 is shown in the form of a roll with an external helical thread formed thereon. The roll 11 may be any desired diameter and is adapted to be driven in a cluster of such rolls spaced to form a passline between the rolls which is correct for the size of the work to be externally threaded. In this embodiment of the invention a single thread is developed on the surface of the roll 11 consisting of a plurality of turns in the form of a continuous helix. Depending upon the diameter of the dies in relation to the work, multiple helical thread configurations may also be employed. For simplicity of illustration, however, the invention is illustrated with respect to a single continuous helical thread on the roll with the understanding that the invention can be applied to multiple helical thread configurations and to annular thread configurations as illustrated in FIG. 6 and described below.

The roll 11 comprises a starting relief section 12, a dwell section 13 and a finish relief section 14. The points of transition between the sections l2, l3 and 14 are indicated approximately by dot and dash lines in FIG. 1. Also shown in FIG. 1 is a portion of a cylindrical work piece 15 which is progressing through a set of rolls 11 with rolling forces applied radially inwardly by the rolls in balanced relation to the work piece such that the desired external thread formation is rolled into the metal surface of the work piece. It will be understood that the rolling operation takes place with the die 11 and the other identical dies (not shown) each rotating about its central axis, not shown, which may or may not be parallel to the direction of motion of the work piece indicated by the arrow in FIG. 1. The roll axis about which the roll 11 rotates is also taken as the axis of reference for the angles of the various portions of the helical thread on the surface of the die 11 hereinafter described.

The dwell section 13 of the die 11 is provided with a full size thread having crests 16 and roots 17 which correspond in size and pitch to the desired finished thread formation for the work 15. The length of the dwell section 13 includes a plurality of complete thread turns and operates in a known manner to finish the external thread rolled onto the work piece into the final desired size and tolerance dimension.

The envelope of the crests 16 is parallel to the axis of rotation of the roll 11 and the envelope of the roots 17 is also parallel to the axis of rotation of the roll 11. Thus the dwell section serves to finish form the threads on the work piece 15 with the desired thread configuration having roots and crests that are uniformly distant from the axis of the round work piece 15 due to the action of the die crests 16 and roots 17 which are equidistant from the axis of rotation of the roll 11.

The finish relief section 14 has a plurality of full size thread turns having crests 18 and roots 19 and differs from the dwell section 13 in that the envelope of the roots l9 tapers towards the axis of rotation of the roll 11. Similarly, the envelope of the crests 18 tapers toward the axis of the roll 11 and is in fact parallel with the envelope of the roots 19. Thus with respect to the work which passes through the set of dies 11 the finish relief section 14 tapers with respect to both crests 18 and roots l9 and moves gradually away from the corresponding roots and crests of the finished thread leaving the dwell section 13 thereby to free the work from further deformation action of the die and to permit it to exit from the free end of the finish relief section 14.

The starting relief section 12, in accordance with the present invention, provides generally a continuation of the threads from the dwell section 13 with roots 21 for all thread turns in the starting relief section 12 spaced equidistant from the axis of rotation of the roll 11. Thus the envelopes of the roots 21 of the starting section and the roots 17 of the dwell section are of the same diameter and are aligned parallel with the axis of rotation of the roll 11. The crests 22 are flat in longitudinal section and coincide with the surface of a right circular cone which tapers at an angle a relative to the axis of rotation of the roll 11. In this embodiment the flat surfaces of the crests 22 coincide with the envelope angle and thus the general shape of the starting relief section is simply a uniformly tapered reduction in diameter. This taper produces the flat crests 22 which progress from zero width at the dwell section 13 to increasingly wider crests 22 and more shallow depressions to the roots 21 which, as previously stated, remain uniformly spaced from the axis of rotation of the die 11. The angle of the taper is such that the entry end of the die has a diameter which, when mounted in a cluster, provides an entrance of greater diameter than the diameter of the cylindrical work piece 15 as it enters the die. Thus the crests 22 contact the work piece at some intermediate point suggested at 23 on the tapered surface of the entry relief section 12 and as contact is made the internal metal flow in the work 15 is initiated.

Referring now to FIG. 2, the details of the development of the thread in work piece 15 are shown. As indicated, the flat crests 22 develop metal flow in the surface of the work piece 15 which progresses into the roots 21 of the die 12 without any bifurcation or tendency to produce a crevice in the developing thread structure on the work 15. The metal flows steadily toward the root of the die thread as the crest moves into the work without producing the cupped formation characteristic of the prior art which caused the creviced crest. As the work approaches the end of the starting relief section 12, a rounded crest formation 24 free of any crevice develops on the work thread which in the dwell section 13 is completed as a finished thread.

Referring now to FIG. 3, a modified formof the invention is shown which permits the starting relief section to be reduced in length relative to the embodiment shown in FIG. 1. In FIG. 3 a roll 11' having a truncated starting relief section 32, a uniform dwell section 33 and a finish relief section 34 is shown. The construction of the dwell section 33 is substantially the same as that previously described for dwell section 13 and the finish relief section 34 is substantially identical with the finish relief section previously described forsection 14 of FIG. 1.

The starting relief section 32 has threads having roots 31 which are at a uniform radius from the axis of rotation of the roll 11'. Thus the envelope of roots 3] is cylindrical and is an extension of the cylinder that locates the roots in the threads 35 of the dwell section 33. Crests 34 of the threads in the starting relief section, however, are, in longitudinal section, flat surfaces having a predetermined angle relative to the axis of rotation which generally corresponds with the desired taper angle a defined in connection with the embodiment shown in FIG. 1. The envelope of these crests 34, however, has a greater angle B than the angle a of the actual crest surfaces.

Referring to FIG. 4, the development of the metal fiow to produce the finished thread is indicated for work piece 15' and in addition construction lines have been added to indicate the relationship between the angle a the surfaces of crests 34 make with the axis in relation to the general taper angle B of the envelope of the crests. Thus the angle a is indicated as generally the same as shown with respect to FIG. 1 but the angle of the envelope is indicated by angle B which is a larger angle than the angle a.

With this construction, the entry relief section threads 34 progress from left to right starting at the dwell section 33 with a stair step character that uniformly reduces the distance of the crests 34 from the axis of rotation according to the angle B but maintains the angle aof the crest surfaces constant with respect to axis of rotation. This angular relation is clearly illustrated in the diagram shown in FIG. 5 which presents the construction lines involved in the stair step thread position and the flat crest angle for each thread. The upper line 36 of the angle a is parallel to the flat crest surface 34. The upper line 38 of the angle ,8 touches only the leading edges 40 of crests 34. It is therefore apparent that a plane coincident with any flat crest surface 34, if extended, will lie outside of the next adjacent crest 34 toward the starting end of the die and will be inside the next adjacent crest 34 away from the starting end.

With reference now to FIG. 6, a external thread rolling die according to an alternative embodiment of the invention is shown in the form of a roll with a series of annular threads and is generally indicated as 41. The roll 41 comprises a starting relief section 42, a dwell section 43 and a finish relief section 44, which sections are substantially identical to sections 12, 13 and 14 of the embodiment shown in FIGS. 1 and 2 with the exception that an annular, rather than a helical, thread is provided. Thus the envelopes of dwell section crests 46 and roots 47 are parallel to the axis of the roll 41; the envelopes of finish relief section crests 48 and roots 49 taper toward the axis of the roll 41; the envelope of starting relief section roots 51 is an extension of the dwell section root 47 envelope and is parallel to the axis of the roll 41; and starting relief crests 52 are flat in longitudinal section and taper at the angle a to the axis of the roll 41.

In operation, a cluster of the rolls 41 are rotatably mounted and driven at an angle to the acid of the work piece 15 to drive the work 15 and the external thread on the work piece 15 develops in the manner described above with reference to FIG. 2. It should be appreciated that the roll 41 could also be provided with a truncated starting relief section similar to that shown in FIGS. 3, 4 and 5. The tool, according to the embodiments of FIGS. 1 and 6, may be manufactured by well known techniques and, in particular, the starting relied sections 12 and 42 and the dwell sections 13 and 43 may initially be formed as a uniform helical or annular thread having a constant root depth and constant spacing from the axis of rotation of the roll. The starting relief sections 12 and 42 may then be tapered uniformly as by grinding with the crests 22 and 52 having the same angle for their surfaces as the overall taper angle a for the entire section.

The manufacture of the embodiment shown in FIG. 3 corresponds in all respects to that described for FIGS. 1 and 6 except that the starting relief section requires that the crests be ground off with an overall taper angle B for the section and with the crest surfaces 34 of each thread provided as flat areas which make a different angle a with the axis than the overall taper angle B. As previously indicated, this angle a may be substantially the same angle which is used in connection with the embodiments of FIGS. 1 and 6. In each case this angle a is preferably between 1%" to 5 and in any event not over 12. For the embodiment shown in FIGS. 3, 4 and 5, the angle B will always be greater than the angle a and may be, for example, twice the angle a.

The dies heretofore described carry out a novel method in the formation of exterior threads on a cylindrical work piece. The relieved helical thread on the starting section of the die initially engages the cylindrical work piece along a relatively wide slightly sloping surface. As the dies and the work rotate, the applied force enters into the work piece to a greater depth with the wide slightly sloping surface of the tool decreasing in width as the remote sides of the tool that slope equally in opposite directions toward each other enter into the work piece to an increasing depth. The metal is displaced both laterally and outwardly beyond the original diameter of the work piece. The depth of entry of the tool into the work piece terminates when the two remote sides of the tool intersect each other and the intermediate slightly sloping surface between the two sides has disappeared. The extent of the entry of the tool into the work piece is equal to only half the depth of the finished thread. The metal of the work piece that is displaced is displaced at a decreasing rate and that part moving outwardly enters into a helical space in the tool beyond the original circumference of the work piece which space has a greater circumference than the circumference of the space from which the metal has been displaced by the tool. Thus, the metal entering into the exterior area provided by the tool beyond the original surface of the work piece enters into a space of greater volume than that from which it was displaced. Hence, the metal forming the outer half of thread beyond the pitch line may be of slightly less density than the base portion of the thread adjacent the root. From the foregoing, it will be understood that the tool performs a method in which the threads are created by displacement of metal at a diminishing rate and with that part of the thread beyond the pitch line being of no greater density than the density of the root portions of the thread. This method is to be distinguished from the prior art disclosures relating to the rolling of internal threads in which the threads are created by compression of the metal.

We claim:

1. A through feed thread rolling die for producing metal flow on the exterior surface of a cylindrical metal work piece to form a continuous thread configuration free of a crevice along the thread crest, comprising a roll formed with an external thread having a starting section, dwell section and a finish relief section, said starting and dwell sections having a cylindrical root envelope whose axis coincides with the axis of said roll, said starting section being relieved and having a plurality of thread turns with flat crests which form a constant predetermined angle with the roll axis, said starting section thread crests progressively decreasing in width and increasing in diameter as they approach said dwell sections, said starting section thread crests commencing at the starting end with a crest height below the mean height of the threads of said dwell section, the outer envelope of said starting section defining a truncated cone which 18 tangent to the leading side edge of said flat crests, the angle of said outer envelope to the said roll axis being greater than the said constant predetermined angle of said flat crests with the roll axis, said dwell section having a fully formed thread of uniform dimensions throughout its length; and said finish section tapering to a reduced diameter thereby to provide adequate relief for the completed thread of the workpiece as it leaves the said dwell section.

2. A through feed thread rolling die as set forth in claim 1 in which said constant predetermined angle of said flat crests with the roll axis is not more than 336. 

1. A through feed thread rolling die for producing metal flow on the exterior surface of a cylindrical metal work piece to form a continuous thread configuration free of a crevice along the thread crest, comprising a roll formed with an external thread having a starting section, dwell section and a finish relief section, said starting and dwell sections having a cylindrical root envelope whose axis coincides with the axis of said roll, said starting section being relieved and having a plurality of thread turns with flat crests which form a constant predetermined angle with the roll axis, said starting section thread crests progressively decreasing in width and increasing in diameter as they approach said dwell section, said starting section thread crests commencing at the starting end with a crest height below the mean height of the threads of said dwell section, the outer envelope of said starting section defining a truncated cone which is tangent to the leading side edge of said flat crests, the angle of said outer envelope to the said roll axis being greater than the said constant predetermined angle of said flat crests with the roll axis, said dwell section having a fully formed thread of uniform dimensions throughout its length; and said finish section tapering to a reduced diameter thereby to provide adequate relief for the completed thread of the workpiece as it leaves the said dwell section.
 2. A through feed thread rolling die as set forth in claim 1 in which said constant predetermined angle of said flat crests with the roll axis is not more than 3 1/2 * . 